1. Field of the Invention
This invention relates to a double-tone printer in which an image is printed in light color ink and dark color ink which are overlapped with each other, and more particularly to a double-tone printer such as a digital stencil printer for professional use which prints a high quality black-and-white image and a method of making a density correction curve for use in a density correction processing of a digital image with double-tone printing.
2. Description of the Related Art
For example, a digital stencil printer has been widely used because of its high printing speed and low cost. In the digital stencil printer, a stencil master is made by bringing a thermal head, provided with an array of heater elements which are selectively heated according to an image to be printed, into contact with a heat-sensitive master material so that the parts of the heat-sensitive master material brought into contact with the heated heater elements are perforated and become ink-permeable and form picture elements.
In the digital stencil printer, gradation is controlled by controlling the density of the dots (picture elements). Accordingly, in a low density region, grain coarseness appears in the image due to a low dot density. Further since it is hard to uniform the size and shape of the dots, rough impression appears in a low density region and an intermediate density region.
As a method of printing a high quality black-and-white image, there has been a method known as double-tone printing. In the double-tone printing, two kinds of stencil masters are made on the basis of an original and printing is effected by use of the stencil masters with light color ink and dark color ink which are similar in hue. The light color ink is generally gray ink and the dark color ink is generally black ink. In this system, to faithfully reproduce the gradation of the original is important, and gradation of an intermediate density region to a high density region which has been hard to reproduce in conventional black-and-white printing can be expressed and depth of an image can be obtained. (See, for instance, xe2x80x9cAll of Scannerxe2x80x9d: Nihon Gijutsu Insatu Kyoukai, from P78 on.) It is expected that when the technique of double-tone printing is applied to the digital stencil printing, appearance of grain coarseness in a low density region and appearance of rough impression in an intermediate density region can be suppressed.
As methods of making two kinds of stencil masters in the double-tone printing, there have been known those in which a plate making camera or a scanner is used and the two kinds of stencil masters are made by use of different tone curves.
However, since the two different tone curves are empirically set by a printer operator, the steps of making stencil masters and trial printing must be repeated until a desired density of the image is obtained.
To the contrast, there are methods in which the density obtained when the light color ink image and the dark color ink image are overlapped is theoretically obtained without actually effecting printing. For example, there has been known a method in which Neugebauer equation (Neugebauer, H. E.: Z. wiss. Phot., 34(4)(1937), p.73), an equation obtained by combining the proportions of areas of the respective color inks with mean additive blending, is used and the print density for given proportions of areas of the respective color inks is obtained.
However, in this method, there are enormous number of combinations of proportions of the color ink areas for a given print density. Accordingly, it is impossible to determine the tone curves for the respective inks on the basis of a target print density by use of Neugebauer equation. That is, there are many solutions for the equation unless one of the tone curves is determined.
Accordingly, conventionally, the proportion of the area of gray ink ag and the proportion of the area of black ink ak are related to each other to satisfy the following formula (1)
ag=Kxc2x7akxe2x80x83xe2x80x83(1)
wherein K represents a value depending on a target reflectance, so that only one solution is obtained.
When the technique of the known double-tone printing is applied to the digital stencil printing as it is, the advantages of the stencil printing that the printing speed is high and printing cost is low are sacrificed due to the fact that the steps of making stencil masters and trial printing must be repeated though the quality of the printed matter is improved.
When the aforesaid formula (1) is used, the problem of the printing time and the printing cost is overcome to some extent. However this approach is disadvantageous in that since both the light color ink (gray ink) and the dark color ink (black ink) must be constantly used according to the formula (1), black ink must be included in a predetermined proportion of area even in a low density region and accordingly the picture element density is still low in such a low density region, whereby grain coarseness cannot be sufficiently removed.
In view of the foregoing observations and description, the primary object of the present invention is to provide a double-tone printer in which a print density suitable for suppressing appearance of grain coarseness in a low density region and appearance of rough impression in an intermediate density region can be quickly determined without repetition of trial printings and a high quality printing can be easily effected.
Another object of the present invention is to provide a method of making density correction curves for double-tone printing rationally in a routinized manner.
In accordance with a first aspect of the present invention, there is provided a double-tone printer comprising, as shown in FIG. 1, an image input means which inputs an image signal made up of a series of image signal components each representing a picture element on an original,
a stencil master making signal producing means which converts the image signal into a density signal made up of a series of density signal components each representing a print density of a picture element according to a density correction curve for a light color and makes a stencil master making signal for the light color by binary-coding the density signal components and which converts the image signal into a density signal made up of a series of density signal components each representing a print density of a picture element according to a density correction curve for a dark color and makes a stencil master making signal for the dark color by binary-coding the density signal components,
a stencil master making means which makes a light color stencil master and a dark color stencil master respectively on the basis of the stencil master making signal for the light color and the stencil master making signal for the dark color, and
a printing means which prints the images of the respective stencil masters on a printing paper in light color ink and dark color ink so that the printed images in the respective color inks overlap each other,
wherein the improvement comprises that
the density correction curve for the light color gives a print density which increases with increase in the value of the image signal component in a lower range where the values of the image signal components are lower than a threshold value and is fixed at a maximum value in a higher range where the values of the image signal components are not lower than the threshold value, and
the density correction curve for the dark color gives a print density which is fixed at a minimum value in the lower range and increases with increase in the value of the image signal component in the higher range.
In the printer with this arrangement, out of the image obtained from the stencil master making signal for the light color, the region which can be expressed by the light color ink is printed solely in the light color ink and the region which cannot be expressed solely by the light color ink is printed in both the light color ink and the dark color ink.
Further when the density correction curve for the light color and that for the dark color are made on the basis of a test chart printed by use of said stencil master making means and said printing means, higher quality printing can be realized according to change in the printing condition and the like.
Though the density correction curves may be calculated by a personal computer and the like and the result of the calculation may be input into the printer, it is preferred that the printer be provided therein with function of making the density correction curves.
In accordance with a second aspect of the present invention, there is provided a method of making the density correction curves for the light color and the dark color which are used in a double-tone printer, the method comprising the steps of
obtaining a gray scale image signal which represents a gray scale having a plurality of regions which are different from each other in density and is made up of a series of image signal components,
obtaining a solid image signal which represents a solid image made up of picture elements which are the same in density and is made up of a series of image signal components which are the same in value,
printing an image of a gray scale in light color ink on a first printing paper according to the gray scale image signal, an image of a gray scale in dark color ink on a second printing paper according to the gray scale image signal, and solid images on a third printing paper in the light color ink and the dark color ink according to the solid image signal so that the images printed in the light color ink and the dark color ink overlap each other,
measuring the reflectances of the images on the first to third printing papers, thereby obtaining a light color ink reflectance curve which represents the relation between the reflectance of each regions of the image of the gray scale printed on the first printing paper in the light color ink and the value of the image signal components corresponding to the region, a dark color ink reflectance curve which represents the relation between the reflectance of each regions of the image of the gray scale printed on the second printing paper in the dark color ink and the value of the image signal components corresponding to the region, and the reflectance given by the overlapping solid prints in the light color ink and the dark color ink,
obtaining a target print reflectance curve, which represents a desired relation between the values of the image signal components and the reflectance of a printed image obtained by the double-tone printing on the basis of said reflectance curves and the reflectance,
obtaining a density correction curve for the light color, which, in order to express only in the light color ink a range of reflectance which can be expressed only in the light color ink, gives a print density which increases with increase in the value of the image signal component in a lower range where the values of the image signal components are lower than a threshold value and is fixed at a maximum value in a higher range where the values of the image signal components are not lower than the threshold value, on the basis of the target print reflectance curve and the light color ink reflectance curve,
calculating the proportion of the area of the dark color ink and the reflectance of the dark color ink corresponding to the proportion of the area of the dark color ink on the basis of the density correction curve for the light color and the target print reflectance curve, and
obtaining a density correction curve for the dark color, which, in order to express in both the light color ink and the dark color ink a range of reflectance which cannot be expressed only in the light color ink, gives a print density which is fixed at a minimum value in the lower range and increases with increase in the value of the image signal component in the higher range on the basis of the calculated reflectance of the dark color ink and the dark color ink reflectance curve.
For example, the image signal may be a digital image signal obtained by digitizing an analog image signal read out from an original such as a photography or a document by an image scanner, or a digital image signal representing an image made as a graphic data by a personal computer or the like.
The reflectance of the image may be measured by use of a calorimeter or by use of an image scanner for reading out the image on the original as a substitute for the calorimeter.
Calculation of the proportion of the area of the dark color ink and the reflectance of the dark color ink corresponding to the proportion of the area of the dark color ink can be effected by use of equations such as Neugebauer equation which have been known in the field of double-tone printing.
In accordance with a third aspect of the present invention, there is provided a double-tone printer having therein a system for making the density correction curves for the light color and the dark color according to the method of the second aspect. That is, the double-tone printer in accordance with the third aspect of the present invention comprises
an image input means which inputs a digital image signal made up of a series of image signal components each representing a picture element on an original,
a tone curve storage means which stores a tone curve representing the relation between the value of each image signal component of the digital image signal and the print density to be used in double-tone printing,
a density correction means which corrects the densities of the picture elements represented by the image signal components of the digital image signal according to a predetermined density correction curve, and
a stencil master making/printing means which binary-codes the densities of the picture elements represented by the corrected image signal, makes a stencil master on the basis of the binary-coded image signal and makes a print by use of the stencil master,
wherein the improvement comprises
a gray scale printing means which prints an image of a gray scale in light color ink on a first printing paper according to a gray scale image signal which represents a gray scale having a plurality of regions which are different from each other in density and is made up of a series of image signal components, and prints an image of a gray scale in dark color ink on a second printing paper according to the gray scale image signal,
a solid image printing means which prints solid images on a third printing paper in the light color ink and the dark color ink according to a solid image signal which represents a solid image made up of picture elements which are the same in density and is made up of a series of image signal components which are the same in value so that the images printed in the light color ink and the dark color ink overlap each other,
a colorimetry means which measures the reflectances of the images on the first to third printing papers,
a light color ink reflectance curve making means which makes a light color ink reflectance curve, which represents the relation between the reflectance of each regions of the image of the gray scale printed on the first printing paper in the light color ink and the value of the image signal components corresponding to the region, on the basis of the measured reflectance of the image on the first printing paper,
a dark color ink reflectance curve making means which makes a dark color ink reflectance curve, which represents the relation between the reflectance of each regions of the image of the gray scale printed on the second printing paper in the dark color ink and the value of the image signal components corresponding to the region, on the basis of the measured reflectance of the image on the second printing paper,
a target print reflectance curve making means which makes a target print reflectance curve, which represents a desired relation between the values of the image signal components and the reflectance of a printed image obtained by the double-tone printing, on the basis of said reflectance curves and the reflectance,
a light color density correction curve making means which makes a density correction curve for the light color, for expressing only in the light color ink a range of reflectance which can be expressed only in the light color ink, on the basis of the target print reflectance curve and the light color ink reflectance curve,
a calculating means which calculates the proportion of the area of the dark color ink and the reflectance of the dark color ink corresponding to the proportion of the area of the dark color ink on the basis of the density curve for the light color and the target print reflectance curve, and
a dark color density correction curve making means which makes a density correction curve for the dark color, for expressing in both the light color ink and the dark color ink a range of reflectance which cannot be expressed only in the light color ink, on the basis of the calculated reflectance of the dark color ink and the dark color ink reflectance curve,
wherein the density correction means corrects the densities of the picture elements represented by the image signal components of the digital image signal according to one of the density correction curves for the light color and the dark color so that the range of density which can be expressed only in the light color ink is expressed only in the light color ink and the range of density which cannot be expressed only in the light color ink is expressed in both the light color ink and the dark color ink.
In accordance with a fourth aspect of the present invention, there is provided a method of making the density correction curves for the light color and the dark color which are used in a double-tone printer, the method comprising the steps of
printing an image of a gray scale on a printing paper in light color ink according to a gray scale image signal which represents a gray scale having a plurality of regions which are different from each other in density and is made up of a series of image signal components and measuring the reflectance of the image on the printing paper, thereby obtaining a light color ink reflectance curve which represents the relation between the reflectance of each regions of the image of the gray scale printed on the printing paper in the light color ink and the value of the image signal components corresponding to the region,
printing a solid image on a printing paper in the light color ink according to a solid image signal which represents a solid image made up of picture elements which are the same in density and is made up of a series of image signal components which are the same in value, printing on the printing paper in the dark color ink an image of a gray scale according to the gray scale image signal so that the image printed in the dark color ink overlap the image printed in the light color ink, and measuring the reflectance of the image on the printing paper, thereby obtaining a lapping dark color ink reflectance curve which represents the reflectance of each regions of the image of the gray scale printed on the printing paper in the dark color ink overlapping the light color ink and the value of the image signal components corresponding to the region,
obtaining a target print reflectance curve, which represents a desired relation between the values of the image signal components and the reflectance of a printed image obtained by the double-tone printing, on the basis of said reflectance curves,
obtaining a density correction curve for the light color, which, in order to express only in the light color ink a range of reflectance which can be expressed only in the light color ink, gives a print density which increases with increase in the value of the image signal component in a lower range where the values of the image signal components are lower than a threshold value and is fixed at a maximum value in a higher range where the values of the image signal components are not lower than the threshold value, on the basis of the target print reflectance curve and the light color ink reflectance curve, and
obtaining a density correction curve for the dark color, which, in order to express in both the light color ink and the dark color ink a range of reflectance which cannot be expressed only in the light color ink, gives a print density which is fixed at a minimum value in the lower range and increases with increase in the value of the image signal component in the higher range, on the basis of the target print reflectance curve and the lapping dark color ink reflectance curve.
That is, in the method of the second aspect, the density correction curve for the dark color is made by use of the density correction curve for the light color which has been made before. To the contrast, in the method of the fourth aspect, the density correction curve for the dark color is made on the basis of the measured reflectance of the image of a gray scale in the dark color ink over a solid printing in the light color ink.
In accordance with a fifth aspect of the present invention, there is provided a double-tone printer having therein a system for making the density correction curves for the light color and the dark color according to the method of the fourth aspect. That is, the double-tone printer in accordance with the fifth aspect of the present invention comprises
an image input means which inputs a digital image signal made up of a series of image signal components each representing a picture element on an original,
a tone curve storage means which stores a tone curve representing the relation between the value of each image signal component of the digital image signal and the print density to be used in double-tone printing,
a density correction means which corrects the densities of the picture elements represented by the image signal components of the digital image signal according to a predetermined density correction curve, and
a stencil master making/printing means which binary-codes the densities of the picture elements represented by the corrected image signal, makes a stencil master on the basis of the binary-coded image signal and makes a print by use of the stencil master,
wherein the improvement comprises
a gray scale printing means which prints an image of a gray scale in light color ink on a first printing paper according to a gray scale image signal which represents a gray scale having a plurality of regions which are different from each other in density and is made up of a series of image signal components,
a lap printing means which prints a solid image on a second printing paper in the light color ink according to a solid image signal which represents a solid image made up of picture elements which are the same in density and is made up of a series of image signal components which are the same in value, and prints on the second printing paper in the dark color ink an image of a gray scale according to the gray scale image signal so that the image printed in the dark color ink overlap the image printed in the light color ink,
a colorimetry means which measures the reflectances of the images on the first and second printing papers,
a light color ink reflectance curve making means which makes a light color ink reflectance curve, which represents the relation between the reflectance of each regions of the image of the gray scale printed on the first printing paper in the light color ink and the value of the image signal components corresponding to the region, on the basis of the measured reflectance of the image on the first printing paper,
a lapping dark color ink reflectance curve making means which makes a lapping dark color ink reflectance curve, which represents the reflectance of each regions of the image of the gray scale printed on the printing paper in the dark color ink overlapping the light color ink and the value of the image signal components corresponding to the region, on the basis of the measured reflectance of the image on the second printing paper,
a target print reflectance curve making means which makes a target print reflectance curve, which represents a desired relation between the values of the image signal components and the reflectance of a printed image obtained by the double-tone printing, on the basis of said reflectance curves,
a light color density correction curve making means which makes a density correction curve for the light color, for expressing only in the light color ink a range of reflectance which can be expressed only in the light color ink, on the basis of the target print reflectance curve and the light color ink reflectance curve, and
a dark color density correction curve making means which makes a density correction curve for the dark color, for expressing in both the light color ink and the dark color ink a range of reflectance which cannot be expressed only in the light color ink, on the basis of the target print reflectance curve and the lapping dark color ink reflectance curve,
wherein the density correction means corrects the densities of the picture elements represented by the image signal components of the digital image signal according to one of the density correction curves for the light color and the dark color so that the range of density which can be expressed only in the light color ink is expressed only in the light color ink and the range of density which cannot be expressed only in the light color ink is expressed in both the light color ink and the dark color ink.
In the double-tone printers of the present invention, since the low density region is printed only in the light color ink, the density of the picture elements can be higher as compared with in the conventional double-tone printer where the low density is printed both the light color ink and the dark color ink, whereby appearance of grain coarseness in a low density region can be suppressed. Further since the density range which cannot be expressed only in the light color ink is constantly printed in both the light color ink and the dark color ink and there is no density range which is printed only in the dark color ink, appearance of rough impression in an intermediate density region can be suppressed. Further even if the image printed in the light color is somewhat shifted from the image printed in the dark color, quality of the printed image is not largely affected, whereby load on the operator is reduced and a high quality printing can be effected quickly and surely.
Further in accordance with the method of making density correction curves of the present invention, since test printing of a digital gray scale and the like is measured by a colorimetry and the like, the measured reflectance is related to the image signal component for a picture element and the density correction curve is made on the basis of the measured relation between the image signal component and the reflectance so that a desired tone curve is obtained, the density correction curves for double-tone printing can be made rationally in a routinized manner even by an unskilled operator.